A fuel cell is a sort of electric generator which generates electric energy by electrochemically oxidizing fuels such as hydrogen and methanol and has lately attracted attention as a clean energy source. The fuel cell is classified into: a phosphoric acid type, a molten carbonate type, a solid oxide type, a solid polyelectrolyte type or the like depending on the kind of electrolyte used. Among these the solid polyelectrolyte type of fuel cell is expected to be widely applied as a power source for electric vehicles or the like because of the low standard operating temperature of 100° C. or less and the high energy thereof.
The solid polyelectrolyte type of fuel cell is basically composed of an ion exchange membrane and a pair of gas diffusion electrodes bonded to both sides thereof. It generates electricity by supplying hydrogen to one electrode and oxygen to the other electrode and connecting both electrodes to an external load circuit. More specifically, a proton and an electron are generated in the hydrogen side electrode. The proton migrates through the ion exchange membrane to the oxygen side electrode, and then reacts with oxygen to form water, while the electron flows through a lead wire from the hydrogen side electrode and discharges electric energy in the external load circuit and then arrives at the oxygen side electrode through another lead wire, contributing to the above water-forming reaction. Although required characteristics of the ion exchange membrane is high ion conductivity inthe first place, high water content and high water dispersibility in addition to the ion conductivity are also important required characteristics because the proton is considered to be stabilized by hydration of a water molecule when migrating through the ion exchange membrane. In addition, since the ion exchange membrane also functions as a barrier to prevent direct reaction of hydrogen and oxygen, low gas permeability is required. Furthermore, the properties such as chemical stability to resist a strongly acidic atmosphere during the fuel cell operation and mechanical strength to meet the requirements for thinner membrane are also necessary.
An ion exchange fluorocarbon resin is widely employed as a material for the ion exchange membrane to be used for the solid polyelectrolyte type of fuel cell, because of its high chemical stability. Particularly “Nafion(registered trademark)” made by DuPont Co. having a perfluorocarbon as a main chain and a sulfonic acid group at an end of a side chain is widely used. Although such ion exchange fluorocarbon resin generally has well-balanced properties as a solid polyelectrolyte material, further improvements in the properties thereof have been required to advance the practical use of said fuel cells.
With regard to the long term durability of fuel cell, for instance, a participation of mechanical strength in a high temperature and a high humidity state is suggested as a factor thereof. Several methods to improve the mechanical strength of the ion exchange membrane have been conventionally proposed, and as an example, JP-A-53-149881 discloses “A reinforced cation exchange resin characterized in that a fibrillated tetrafluoroethylene polymer is contained in a cation exchange resin forming a cation exchange resin membrane”. The production method described in said application is characterized by mixing a) an ion exchange resin precursor or a swollen material thereof with trichlorotrifluoroethane, and b) fine powder or an emulsified aqueous dispersion of a tetrafluoroethylene polymer (hereinafter, referred to as PTFE) obtained by an emulsion polymerization or a suspension polymerization, then melt kneading them to obtain a composition. However, according to said production method, a uniform dispersion of PTFE is practically difficult, and results in some problems causing a very poor quality of sheet, such as PTFE agglomerate formed inside the membrane and an uneven surface formed on the sheet in an extrusion sheet molding. This is not only a problem of the quality of sheet surface, but also suggests an essential problem involved in the conventional technology that a certain amount of the added PTFE remains inside the PTFE agglomerate to be wasted without being fibrillated.
In addition, JP-B-63-61337 discloses “A method for making thinner a membrane consisting of a fluorine-containing ion exchange resin containing an uniformly dispersed fibrillated fluorocarbon resin fiber by stretching at a specified temperature”. However, as in the former application, this method also has such problems that PTFE agglomerate tends to be formed inside the membrane and a sheet with an uneven surface tends to be formed. Since it is difficult to produce a thin membrane in particular when such uneven surface is formed, for instance, said application describes that the membrane is subjected to a smoothing treatment using a roll press or the like prior to thinning by stretching. Further, JP-B-61-16288 discloses a roll press method for such a purpose. As described above, the conventional technologies disclosed in JP-A-53-149881, JP-B-63-61337 and JP-B-61-16288 and the like are limited to mere addition of PTFE, and therefore have not been accepted as an industrially useful technology for an ion exchange membrane for a fuel cell due to difficulties, in particular, in an uniform dispersion, a superior melt molding of PTFE and also an effective use of PTFE.
Furthermore, JP-A-2001-29800 discloses an ion exchange membrane prepared by using an aqueous dispersion containing at least three essential components of fine particles of fluorocarbon resin, an ion exchangeable polymer and a fluorine-containing surface-active substance. This application further discloses a method for preparing such an aqueous dispersion by mixing three essential components of an aqueous suspension of fine particles of fluorocarbon resin, a solution of an ion exchangeable polymer and a solution of a fluorine-containing surface-active substance. Said application describes the use of a solution of the ion exchangeable polymer as a characteristic of the invention, and therefore does not describe anything about washing, which is essential to use a dispersion of an ion exchange fluorocarbon resin precursor. Further, the application does not describe that the aqueous suspension or solution can be obtained directly from a polymerization liquid in each polymerization process without separating or agglomerating solid resin. For instance, judging from the description in the application that a solution of an ion exchangeable polymer is prepared using water or an organic liquid as a solvent, it can be understood that at least the solution of the ion exchangeable polymer is prepared using the ion exchangeable polymer once separated or agglomerated from a polymerization liquid.
The use of the solution of such ion exchangeable polymer has the following problems:    1) It is known that an aqueous suspension of fine particles of fluorocarbon resin is generally less stable under acidic conditions, and a solution of an ion exchangeable polymer is required to be stabilized by, for instance, adding a fluorine-containing surface-active substance as described in said application, because the solution is naturally strongly acidic and easy to agglomerate in mixing;    2) Preparation of an ion exchangeable polymer solution is required to be carried out through many steps such as separation, hydrolysis and dissolution starting from an ion exchange resin precursor, which should be basically obtained in the form of polymerization liquid, and this results in a complicated process and a low productivity;    3) Since the ion exchangeable polymer once isolated or agglomerated is dissolved again, the molecular chains of the polymer becomes entangled though they originally possesses an ideal expanded form after the polymerization. High dispersion with the aqueous suspension of fine particles of the fluorocarbon resin cannot be effected even when the entangled polymer is dispersed in a solvent.